How about enabling players to stand on hitboxes?
my concept:
Primary hitbox: not passable in any way, detatch if player gets drawn away from it (gravity, standing in a lancaster and doing some rolls with players inside=derp experience)
Secondary hitbox: not passable when player has been on a primary hitbox before
Tertiary hitbox: player can call a key event that checks for VK_SHIFT, wich detatches the player from the hitbox.
Speed of the hitbox gets added to the player speed after setting it to 0, after that add the move speed of the player again or disable movement whilst in such a hitbox and listen for WASD instead and do the required stuff.
check for a player touching a primary hitbox every tick for every loaded vehicle, define a variable to check if players can stand on that vehicle or not.

There are many many issues with hitboxes and player movement in Minecraft. The way to do it would be similar to Rails of War, basically allowing the player to alter the position of their seat to simulate movement. Even so, it would be very difficult to implement and you'd basically be writing your own completely new collision system for the vehicles